This invention relates to an FM receiver, and more particularly to a front end circuit adapted to convert the frequency of a high frequency signal such as VHF received by an input antenna of an FM superheterodyne receiver to an intermediate frequency signal.
The front end circuit is required to have an input circuit which converts the antenna impedance into a suitable value, a high frequency filter circuit having a resonance frequency for filtering the output signal of the input circuit, and an amplifier having a suitable gain. As is well known in the art the front end circuit primarily determines those characteristics of the FM receiver such as sensitivity, S/N ratio, selectivity and spurious response, so that the design of this circuit is important. It is essential to design the front end circuit such that its loss should be small as far as possible to have a good sensitivity when the received electric field intensity is large. However, when the received electric field intensity is small or when the receiver is used in a place where interference signals are present, it is necessary to improve the selectivity to suppress unwanted waves as far as possible. More particularly, in order to improve the spurious response characteristic, it is necessary to increase the Q of the resonance circuit of the high frequency filter and to increase the degree of selectivity by increasing the number of stages of the filter. However, this requirement for a good selectivity in the front end is generally a bar to a small loss of the receiver which is necessary to provide a good sensitivity. Further, when the characteristic of the filter is made steep, the phase characteristic in the passband also varies steeply thereby degrading the fidelity characteristic of the VHF signal transmission.
For these reasons, it is necessary to design the circuit of a commercial FM receiver on a suitable compromise that can avoid various troubles described above. Accordingly, it has been necessary to use the front end circuit under conditions wherein respective elements thereof can not fully manifest their desirable characteristics so that it has been impossible to increase the sensitivity up to a limit determined by the noise figure of the amplifier. Moreover it has been impossible to improve the interference eliminating characteristic to a limit determined by the high frequency filter.